A typical complex for urea production includes a syngas/hydrogen production plant, an ammonia production plant and a urea production plant. In a typical complex these plants are built as separate production units. The technologies used in the plants are often provided by different technology providers and design of each of their facilities is usually based on the specification at the battery limit of the previous plant.
The syngas/hydrogen production plant is often based on the conventional steam reforming (SR) process wherein natural gas is reacted with steam to form synthesis gas (H2+CO/CO2). The steam reforming process is endothermic and therefore additional natural gas is combusted in the burners in the reforming unit. The hydrogen is converted with nitrogen into ammonia in the ammonia production unit, while the CO2 is used to react with the ammonia in the urea production unit.
A typical feature of this set up is that the overall process results in a stoichiometric excess of ammonia and consequently a shortage of CO2. This excess of ammonia is typically sold, but this can lead to regulatory and safety issues related to ammonia transportation. Plant owners who desire or are required to consume the excess ammonia on site or wish to expand the capacity of an existing urea plant need to produce additional CO2 to make up of the shortage. In a method known in the art additional CO2 is recovered from flue gas by absorption in a solution, for example an amine solution. Flue gas of the steam reformer burners or flue gas of the auxiliary boiler which produces the steam required for the urea process can be used.
Disadvantages of this known method are that the volume of flue gas to be treated is high while the pressure thereof is typically atmospheric with a low partial pressure of CO2 which leads to large equipment and consequent high capital costs and a need for a significant footprint nearby the steam reformer equipment. The presence of oxygen in the flue gas leads to degradation of the used solution which increases both raw material costs and the cost of disposing of the spent amine solution. Prior to entering the absorption tower, the flue gas needs to be cooled which requires additional equipment and the need for additional cooling water. The pressure drop created by the installation of the absorption system requires also an increase in the internal diameter of the exhaust fan, which may require replacement of the fan which increases both investment and operating cost.
It is therefore desired to provide a method to revamp urea plants by additionally producing CO2, which method does not have the above disadvantages. Particularly, it is desired that this method is economical and does not require large additional equipment or high capital costs.